Re: Add vfio-platform support for ONESHOT irq forwarding?

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 07, 2022, Dmytro Maluka wrote:
> Hi Sean,
> 
> On 7/6/22 10:39 PM, Sean Christopherson wrote:
> > On Wed, Jul 06, 2022, Dmytro Maluka wrote:
> > > This is not a problem on native, since for oneshot irq we keep the interrupt
> > > masked until the thread exits, so that the EOI at the end of hardirq doesn't
> > > result in immediate re-assert. In vfio + KVM case, however, the host doesn't
> > > check that the interrupt is still masked in the guest, so
> > > vfio_platform_unmask() is called regardless.
> > 
> > Isn't not checking that an interrupt is unmasked the real bug?  Fudging around vfio
> > (or whatever is doing the premature unmasking) bugs by delaying an ack notification
> > in KVM is a hack, no?
> 
> Yes, not checking that an interrupt is unmasked is IMO a bug, and my patch
> actually adds this missing checking, only that it adds it in KVM, not in
> VFIO. :)
> 
> Arguably it's not a bug that VFIO is not checking the guest interrupt state
> on its own, provided that the resample notification it receives is always a
> notification that the interrupt has been actually acked. That is the
> motivation behind postponing ack notification in KVM in my patch: it is to
> ensure that KVM "ack notifications" are always actual ack notifications (as
> the name suggests), not just "eoi notifications".

But EOI is an ACK.  It's software saying "this interrupt has been consumed".
 
> That said, your idea of checking the guest interrupt status in VFIO (or
> whatever is listening on the resample eventfd) makes sense to me too. The
> problem, though, is that it's KVM that knows the guest interrupt status, so
> KVM would need to let VFIO/whatever know it somehow. (I'm assuming we are
> focusing on the case of KVM kernel irqchip, not userspace or split irqchip.)
> So do you have in mind adding something like "maskfd" and "unmaskfd" to KVM
> IRQFD interface, in addition to resamplefd? If so, I'm actually in favor of
> such an idea, as I think it would be also useful for other purposes,
> regardless of oneshot interrupts.

Unless I'm misreading things, KVM already provides a mask notifier, irqfd just
needs to be wired up to use kvm_(un)register_irq_mask_notifier().